One way of simplifying the use of various types of appliances which are protected by, for example, passwords, access codes and the like, is to replace the codes with devices which recognize the user on the basis of different physical characteristics, so called biometry. One type of biometrical characteristic is fingerprint information, which can be used to test and either confirm or reject a user's attempt to gain access to an appliance, a premises, etc., referred to as user identity verification. Verification with the aid of fingerprint information is in most cases done by the user having stored his fingerprint information in the equipment in question in advance, by which means a template is created. The information for creating this template can either be input into the appliance which the user will later want to have access to, or via a central appliance.
When the user attempts to gain access to the equipment in question, a biometric device inputs information from the user's finger and compares the input information to the template in order to decide if the user is to be granted access to the equipment or not.
Equipment which verifies with the aid of fingerprint information must satisfy a number of requirements, for example reliability and speed. Verification in portable devices, for example mobile telephones, portable computers, different types of cards, etc., further requires that the equipment must be as small, lightweight and energy-efficient as possible. Another requirement is that the equipment should be as inexpensive as possible.
Known types of devices for verifying a user's identity with the aid of fingerprint information read information from a user's fingerprint and compare this with a template by means of the input information being combined into a representation of the fingerprint, which is then compared with the template which has been stored in advance. A disadvantage of such types of equipment is that they require a great deal of memory, on account of the representation of the fingerprint which is built up. This is also a drawback of known devices for registering said reference information. Memory is expensive, takes up space and requires a great deal of current, which means that known devices are relatively expensive and large. A large use of memory will also slow down the system.
Another parameter which affects cost and takes up space is the surface area of the sensor which is used to read the fingerprint information for the verification. This surface area can be reduced by using what is referred to as a line sensor, in other words a sensor which surface area is too small to read a complete fingerprint without the finger and the sensor moving relative to each other. During verification or registering with the aid of such a sensor, the finger is moved relative to the sensor, and the above-mentioned representation of the fingerprint is built up in a memory on the basis of consecutively input partial areas. Thus, line sensors satisfy the requirements regarding the size and cost of the actual sensor, but the requirement for memory remains. Using a line sensor, the equipment must be able to read information from a moving finger, as distinct from equipment which uses surface-area sensors, against which the finger is simply placed or held.
One example of prior art is SE 515 239, which deals with a method for assembling a composite image of a fingerprint, using a sequence of partial images. This document appears to deal exclusively with assembling an image of, for example, a fingerprint, and suffers from, inter alia, the abovementioned drawback of a large need for memory space.
EP 0813164 B1 discloses a system in which a fingerprint image is collected by relative sliding motion between a finger and the sensor. The sensor is in the form of a line sensor, i.e. a bar that is longer than a finger is wide, but is relatively narrow compared to the length. The sensor thus collects partial images of the fingerprint image and adds them together into a full image, which is stored in a memory. The image of the fingerprint is formed from a matrix of sensor elements.
EP 1317730 B1 discloses a method and a device for checking fingerprints. The method comprises the steps of recording in succession at least two digital images of finger areas and comparing each of the recording images with a reference image that represents at least one previously recorded reference fingerprint from a reference finger areas.
EP 0470530 B1 discloses a fingerprint verification method comprising the steps of performing partial density differentials on a corresponding fingerprint image with respect to two directions, determining a point as a centre of characteristics where an extreme value of a histogram of a corresponding partial differential value occurs in both directions and performing a fingerprint verification by use of the corresponding centre of characteristics.
EP 1150608 B1 discloses a system for checking fingerprints comprising a sensor which is adapted to record a fingerprint made by a fingertip. The surface of a sensor is so small that it can record merely a partial fingerprint. This partial fingerprint is compared with different parts of a complete, previously stored reference fingerprint to determine whether the partial fingerprint originates from the same person as the reference fingerprint.
GB 2310522 B discloses a fingerprint ID system, which compares input fingerprint image data with registered fingerprint image data, where minutiae bulge-line direction patterns and bulge-line curvature patterns are compared, and the matching degree between the fingerprint image data and the registered data is compared with two threshold values for judging if a fingerprint corresponds to registered data. When the matching degree is between the two thresholds, minutiae are compared successively in minutia high-density areas and minutia medium-density areas.
U.S. Pat. No. 3,699,519 A discloses a system in which a fingerprint is observed, a small portion at a time, using a flying spot scanner, whose spot travels along a predetermined path at each position to provide an electrical analogue signal indicative of the nature of the fingerprint at each position. The analogue signal is converted into digital form and temporarily stored in a memory having a plurality of storage elements. The signal stored in the memory is constantly circulated through each of the storage elements to provide for detection of minutiae (i.e. ridge endings, bifurcations, etc.) regardless of their angular orientation. Detecting the occurrence of specified minutia is achieved by sensing the states of selected ones of the storage elements.
U.S. Pat. No. 4,582,985 A discloses a data carrier which comprises an internal processor for processing supplied identification information of the owner during a card verification process, and for generating an acceptance signal as a result of a positive verification. The internal processor includes a verification device which comprises a sensor for sensing a fingertip of the owner and obtaining corresponding finger print line information. When a fingerprint is verified as valid, an acceptance signal is generated.
U.S. Pat. No. 4,607,384 A discloses an arrangement for generating a classification code for a fingerprint having a pattern of ridges, the ridges having a plurality of reference features, which includes overlaying the intersection of a pair of cross-hairs at a first one of the reference features. A line is overlaid from a second one of the reference features to the first reference feature. The cross-hairs are oriented so that the line bisects a quadrant of the cross-hairs. The ridges crossed by predetermined portions of the cross-hairs are counted to generate the classification code.
EP 1330185 B2 discloses a device for biometric verification of an individual's identity by means of fingerprint information, comprising controller and memory means, means for consecutively reading, and storing in said memory means, fingerprint information from partial areas of a finger, and means for extracting, and storing in said memory means, verification data from fingerprint information read from a partial area, means for verification processing of the extracted verification data, and decision-making means for deciding if fingerprint information from a new partial area has been read. At least those points in time when fingerprint information from a new partial area has been read are used by the controller means to update the contents of the memory means, said updating comprising removing fingerprint information from previously read partial areas, the controller means also updating the stored verification data on or before said points in time. In this device, the memory requirement for fingerprint verification is reduced when compared with a system using a full size sensor. The size of the sensor surface is also reduced when compared with a system using a full size sensor.
All of the devices and methods described above require a substantial amount of memory and/or a relatively large sensor surface. There is thus still room for an improved device and an improved method for biometric verification.